It is Sunnah to make these Nafl prayers brief:
1. Two Rak`ahs before Fajr prayer
2. Tahiyyat Al-masjid
3. Starting voluntary night prayers with two brief Rak`ahs
4. Two Rak`ahs following Tawaf.
It is Sunnah to make the following prayers lengthy:
1. The eclipse prayer
2. Qiyam Al-layl
With regard to Sunnah prayers other than that, one should do what helps him be more focused.